>> So very high frequency is not needed?

No, as I mentioned in my second post, you can likely get away with 2MHz

> I=B4ll better use a rare earth metal magnet, as the unit is mobile, and=
 i
also want to keep power requirement down.

> But i don=B4t know anything about this really... how does it work?
> Can i use one sending coil, one magent and a reciever coil placed aroun=
d a
tube in which i want to detect water?  How?


As to the power requirement, Roman Black's suggestion is likely better fo=
r
you there, but here's how it works:

You line up the magnet and the receiver coil so that the magnetic field o=
f
the magnet is perpendicular to the normal vector of the reciever coil.
(I.e. make it so that if you were to run a current through the coil, the
magnetic field inside the coil would be perpendicular to the magnetic fie=
ld
from the magnet at that point)  This is because the nucleii of the water
will be oscillating at 90deg. from the magnetic field.

Now, take the output from the receiver, and ampify it enough to make it
detectable (probably at least 40dB).  At this stage, I would suggest
rectifying it, integrating it (using an opamp integrator) and reading the
output as a DC level.

Now, your receiver coil can be comnimed with your transmitter coil, but i=
f
this is done, you need a T/R (transmit/receive) switch, which couples the
coil to the RF amplifier and signal generator during transmit, and to the
preamplifier/receiver during the receive stage.  To avoid this complexity=
,
you can use two seperate coils, perpendicular to each other.  This also
prevents the need for decoupling, although putting protection diodes on t=
he
pramplifier would be a good idea.

The sequence is this:  (switch to transmit) transmit a bunch, (switch to
receive) receive a bunch, read the level of the output from the integrato=
r,
then reset the integrator and repeat.

Russell, how's that sound to you?

Regards,
--Brendan

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads